This invention relates to a hydrogen-absorbing alloy and to a secondary battery comprising a negative electrode comprising the hydrogen-absorbing alloy.
Hydrogen-absorbing alloy has been noticed as being capable of safely and easily storing hydrogen as an energy source, and also as being useful as an energy exchange material or an energy storage material. Therefore, there have being proposed various applications of hydrogen-absorbing alloy as a new functional raw material. For example, hydrogen-absorbing alloy has been proposed to employ for the storage and transport of hydrogen, the storage and transport of heat, the conversion of heat energy to mechanical energy, the separation and purification of hydrogen, the separation of hydrogen isotope, a battery employing hydrogen as an active material, a catalyst in synthetic chemistry, and a temperature sensor.
Recently, a nickel-hydrogen secondary battery where a hydrogen-absorbing alloy is employed as a negative electrode material has been attracting many attentions as a public-use battery of next generation, because the battery is advantageous in various aspects, e.g. it is high in capacity, highly resistive to over charging and over discharging, capable of performing a high rate charge/discharge, free from environmental pollution, and interchangeable with a nickel-cadmium battery. Accordingly, many attempts have been intensively made at present for the application and actual use of the nickel-hydrogen battery.
As evident from these facts, the hydrogen-absorbing alloy has many possibilities for various applications in view of its physical and chemical characteristics, so that the hydrogen-absorbing alloy is now considered as being one of important raw materials in future industries.
The metal capable of absorbing hydrogen may be in the form of single substance which reacts exothermically with hydrogen, i.e., a metal element capable of forming a stable compound together with hydrogen (for example, Pd, Ti, Zr, V, rare earth elements and alkaline earth elements); or in the form of an alloy comprising an above-mentioned metal alloyed with other kinds of metal.
One of the advantages of the alloying is that the bonding strength between a metal and hydrogen can be suitably weakened so that not only the absorption reaction but also the desorption (releasing) reaction can be performed comparatively easily. Second advantage of the alloying is that the absorption and desorption characteristics of the alloy such as the magnitude of hydrogen gas pressure required for the reaction (equilibrium pressure; plateau pressure), the extent of equilibrium region (plateau region), the change (flatness) of equilibrium pressure during the process of absorbing hydrogen, etc. can be improved. Third advantage of the alloying is the improvement in chemical and physical stability of the alloy.
The composition of the conventional hydrogen-absorbing alloy may be classified into the following types;
(1) a rare earth element type (for example, LaNi.sub.5, MmNi.sub.5, etc.); PA1 (2) a Laves type (for example, ZrV.sub.2, ZrMn.sub.2, etc.); PA1 (3) a titanium type (for example, TiNi, TiFe, etc.); PA1 (4) a magnesium type (for example, Mg.sub.2 Ni, MgNi.sub.2, etc.); and PA1 (5) other types (for example, cluster, etc.). PA1 wherein R1 is at least one element selected from rare earth elements (including Y); M1 is at least one element selected from elements having a larger electronegativity than that of Mg (excluding the elements of R1, Cr, Mn, Fe, Co, Cu, Zn and Ni); and a, b and z are respectively a number satisfying conditions: 0.1.ltoreq.a.ltoreq.0.8, 0&lt;b.ltoreq.0.9, 1-a-b&gt;0, and 3.ltoreq.z.ltoreq.3.8. PA1 wherein R1 is at least one element selected from rare earth elements (including Y); M2 is at least one element selected from the group consisting of Cr, Mn, Fe, Co, Cu and Zn; and a, x and z are respectively a number satisfying conditions: 0.1.ltoreq.a.ltoreq.0.8, 0.ltoreq.x.ltoreq.0.9, and 3.ltoreq.z.ltoreq.3.8. PA1 wherein R1 is at least one element selected from rare earth elements (including Y); M2 is at least one element selected from the group consisting of Cr, Mn, Fe, Co, Cu and Zn; M1 is at least one element selected from elements having a larger electronegativity than that of Mg (excluding the elements of R1, the elements of M1 and Ni); and a, b, x and z are respectively a number satisfying conditions: 0.1.ltoreq.a.ltoreq.0.8, 0&lt;b.ltoreq.0.9, 1-a-b&gt;0, 0&lt;x.ltoreq.0.9, and 3.ltoreq.z.ltoreq.3.8. PA1 wherein R1 is at least one element selected from rare earth elements (including Y); M3 is at least one element selected from the group consisting of Co, Mn, Fe, Al, Ga, Zn, Sn, Cu, Si and B; and a, x and z are respectively a number satisfying conditions: 0.65.ltoreq.a.ltoreq.0.8, 0&lt;x.ltoreq.0.6, and 3.ltoreq.z.ltoreq.3.8. PA1 wherein R1 is at least one element selected from rare earth elements (including Y); T1 is at least one element selected from the group consisting of Ca, Ti, Zr and Hf; M3 is at least one element selected from the group consisting of Co, Mn, Fe, Al, Ga, Zn, Sn, Cu, Si and B; and a, b, x and z are respectively a number satisfying conditions: 0.65.ltoreq.a&lt;0.8, 0&lt;b.ltoreq.0.3, 0.65&lt;(a+b).ltoreq.0.8, 0&lt;x.ltoreq.0.6, and 3.ltoreq.z.ltoreq.3.8. PA1 wherein R1 is at least one element selected from rare earth elements (including Y); M4 is at least one element selected from the group consisting of Mn, Fe, V, Cr, Nb, Al, Ga, Zn, Sn, Cu, Si, P and B; and a, x, y and z are respectively a number satisfying conditions: 0.2.ltoreq.a.ltoreq.0.35, 0&lt;x.ltoreq.0.5, 0.ltoreq.y.ltoreq.0.2, and 3.ltoreq.z.ltoreq.3.8. PA1 wherein R1 is at least one element selected from rare earth elements (including Y); T2 is at least one element selected from the group consisting of Ca, Ti and Zr; M4 is at least one element selected from the group consisting of Mn, Fe, V, Cr, Nb, Al, Ga, Zn, Sn, Cu, Si, P and B; and a, b, x, y and z are respectively a number satisfying conditions: 0.2.ltoreq.a.ltoreq.0.35, 0&lt;b.ltoreq.0.3, 0&lt;x.ltoreq.0.5, 0.ltoreq.y.ltoreq.0.2, and 3.ltoreq.z.ltoreq.3.8. PA1 wherein R1 is at least one element selected from rare earth elements (including Y) but is not La; and a, b and z are respectively a number satisfying conditions: 0.2.ltoreq.a.ltoreq.0.35, 0.01.ltoreq.b&lt;0.5, and 3.ltoreq.z.ltoreq.3.8. PA1 wherein R1 is at least one element selected from rare earth elements (including Y) but is not La; M3 is at least one element selected from the group consisting of Co, Mn, Fe, Al, Ga, Zn, Sn, Cu, Si and B; and a, b, x and z are respectively a number satisfying conditions: 0.2.ltoreq.a.ltoreq.0.35, 0.01.ltoreq.b&lt;0.5, 0.1.ltoreq.x.ltoreq.0.6, and 3.ltoreq.z.ltoreq.3.8. PA1 wherein R2 is two or more kinds of element selected from rare earth elements (including Y), the content of Ce constituting the R2 being less than 20% by weight; T1 is at least one element selected from the group consisting of Ca, Ti, Zr and Hf; M3 is at least one element selected from the group consisting of Mn, Fe, Co, Al, Ga, Zn, Sn, Cu, Si and B; and a, b, x and z are respectively a number satisfying conditions: 0&lt;a.ltoreq.0.5, 0.ltoreq.b.ltoreq.0.3, 0.ltoreq.x.ltoreq.0.9, and 3.ltoreq.z&lt;4. PA1 wherein R3 is two or more kinds of element selected from rare earth elements (including Y); T1 is at least one element selected from the group consisting of Ca, Ti, Zr and Hf; M5 is at least one element selected from the group consisting of Mn, Fe, Al, Ga, Zn, Sn, Cu, Si and B; a, b, x, y and z are respectively a number satisfying conditions: 0&lt;a.ltoreq.0.5, 0.ltoreq.b.ltoreq.0.3, 0.ltoreq.x.ltoreq.0.9, 0&lt;y.ltoreq.0.4, x+y.ltoreq.0.9, and 3.ltoreq.z&lt;4, the content of Ce constituting the R3 being less than m % by weight where m is represented by the following formula (I); EQU m=125y+20 (I) PA1 wherein y is a quantity of Co in the aforementioned general formula (11). PA1 wherein R1 is at least one element selected from rare earth elements (including Y); T1 is at least one element selected from the group consisting of Ca, Ti, Zr and Hf; M6 is at least one element selected from the group consisting of Co, Mn, Fe, Al, Ga, Zn, Sn, Cu, Si, B, Nb, W, Mo, V, Cr, Ta, P and S; and a, b, x and z are respectively a number satisfying conditions: 0.2.ltoreq.a.ltoreq.0.35, 0.ltoreq.b.ltoreq.0.3, 0&lt;x.ltoreq.0.6, and 3.ltoreq.z.ltoreq.3.8; EQU z=-6.times.a+.delta. (II) PA1 wherein .delta. is: 5-0.2.ltoreq..delta..ltoreq.5+0.2. PA1 wherein R1 is at least one element selected from rare earth elements (including Y); T1 is at least one element selected from the group consisting of Ca, Ti, Zr and Hf; M6 is at least one element selected from the group consisting of Co, Mn, Fe, Al, Ga, Zn, Sn, Cu, Si, B, Nb, W, Mo, V, Cr, Ta, P and S; and a, b, x and z are respectively a number satisfying conditions: ##EQU1## PA1 wherein R1 is at least one element selected from rare earth elements (including Y); T1 is at least one element selected from the group consisting of Ca, Ti, Zr and Hf; M6 is at least one element selected from the group consisting of Co, Mn, Fe, Al, Ga, Zn, Sn, Cu, Si, B, Nb, W, Mo, V, Cr, Ta, P and S; and a, b, x and z are respectively a number satisfying conditions: ##EQU2## PA1 wherein R1 is at least one element selected from rare earth elements (including Y); M1 is at least one element selected from elements having a larger electronegativity than that of Mg (excluding the elements of R1, Cr, Mn, Fe, Co, Cu, Zn and Ni); and a, b and z are respectively a number satisfying conditions: 0.1.ltoreq.a.ltoreq.0.8, 0&lt;b.ltoreq.0.9, 1-a-b&gt;0, and 3.ltoreq.z.ltoreq.3.8. PA1 wherein R1 is at least one element selected from rare earth elements (including Y); M2 is at least one element selected from the group consisting of Cr, Mn, Fe, Co, Cu and Zn; and a, x and z are respectively a number satisfying conditions: 0.1.ltoreq.a.ltoreq.0.8, 0&lt;x.ltoreq.0.9, and 3.ltoreq.z.ltoreq.3.8. PA1 wherein R1 is at least one element selected from rare earth elements (including Y); M2 is at least one element selected from the group consisting of Cr, Mn, Fe, Co, Cu and Zn; M1 is at least one element selected from elements having a larger electronegativity than that of Mg (excluding the elements of R1, the elements of M1 and Ni); and a, b, x and z are respectively a number satisfying conditions: 0.1.ltoreq.a.ltoreq.0.8, 0&lt;b.ltoreq.0.9, 1-a-b&gt;0, 0&lt;x.ltoreq.0.9, and 3.ltoreq.z.ltoreq.3.8. PA1 wherein R1 is at least one element selected from rare earth elements (including Y); M3 is at least one element selected from the group consisting of Co, Mn, Fe, Al, Ga, Zn, Sn, Cu, Si and B; and a, x and z are respectively a number satisfying conditions: 0.65.ltoreq.a.ltoreq.0.8, 0&lt;x.ltoreq.0.6, and 3.ltoreq.z.ltoreq.3.8. PA1 wherein R1 is at least one element selected from rare earth elements (including Y); T1 is at least one element selected from the group consisting of Ca, Ti, Zr and Hf; M3 is at least one element selected from the group consisting of Co, Mn, Fe, Al, Ga, Zn, Sn, Cu, Si and B; and a, b, x and z are respectively a number satisfying conditions: 0.65.ltoreq.a&lt;0.8, 0&lt;b.ltoreq.0.3, 0.65&lt;(a+b).ltoreq.0.8, 0&lt;x.ltoreq.0.6, and 3.ltoreq.z.ltoreq.3.8. PA1 wherein R1 is at least one element selected from rare earth elements (including Y); M4 is at least one element selected from the group consisting of Mn, Fe, V, Cr, Nb, Al, Ga, Zn, Sn, Cu, Si, P and B; and a, x, y and z are respectively a number satisfying conditions: 0.2.ltoreq.a.ltoreq.0.35, 0&lt;x.ltoreq.0.5, 0.ltoreq.y.ltoreq.0.2, and 3.ltoreq.z.ltoreq.3.8. PA1 wherein R1 is at least one element selected from rare earth elements (including Y); T2 is at least one element selected from the group consisting of Ca, Ti and Zr; M4 is at least one element selected from the group consisting of Mn, Fe, V, Cr, Nb, Al, Ga, Zn, Sn, Cu, Si, P and B; and a, b, x, y and z are respectively a number satisfying conditions: 0.2.ltoreq.a.ltoreq.0.35, 0&lt;b.ltoreq.0.3, 0&lt;x.ltoreq.0.5, 0.ltoreq.y.ltoreq.0.2, and 3.ltoreq.z.ltoreq.3.8. PA1 wherein R1 is at least one element selected from rare earth elements (including Y) but is not La; and a, b and z are respectively a number satisfying conditions: 0.2.ltoreq.a.ltoreq.0.35, 0.01.ltoreq.b&lt;0.5, and 3.ltoreq.z.ltoreq.3.8. PA1 wherein R1 is at least one element selected from rare earth elements (including Y) but is not La; M3 is at least one element selected from the group consisting of Co, Mn, Fe, Al, Ga, Zn, Sn, Cu, Si and B; and a, b, x and z are respectively a number satisfying conditions: 0.2.ltoreq.a.ltoreq.0.35, 0.01.ltoreq.b&lt;0.5, 0.1.ltoreq.x.ltoreq.0.6, and 3.ltoreq.z.ltoreq.3.8. PA1 wherein R2 is two or more kinds of element selected from rare earth elements (including Y), the content of Ce constituting the R2 being less than 20% by weight; T1 is at least one element selected from the group consisting of Ca, Ti, Zr and Hf; M3 is at least one element selected from the group consisting of Mn, Fe, Co, Al, Ga, Zn, Sn, Cu, Si and B; and a, b, x and z are respectively a number satisfying conditions: 0&lt;a.ltoreq.0.5, 0.ltoreq.b.ltoreq.0. 3, 0.ltoreq.x.ltoreq.0.9, and 3.ltoreq.z&lt;4. PA1 wherein R3 is two or more kinds of element selected from rare earth elements (including Y); T1 is at least one element selected from the group consisting of Ca, Ti, Zr and Hf; M5 is at least one element selected from the group consisting of Mn, Fe, Al, Ga, Zn, Sn, Cu, Si and B; a, b, x, y and z are respectively a number satisfying conditions: 0&lt;a.ltoreq.0.5, 0.ltoreq.b.ltoreq.0.3, 0.ltoreq.x.ltoreq.0.9, 0&lt;y.ltoreq.0.4, x+y.ltoreq.0.9, and 3.ltoreq.z&lt;4, the content of Ce constituting the R3 being less than m % by weight where m is represented by the following formula (I); EQU m=125y+20 (I) PA1 wherein y is a quantity of Co in the aforementioned general formula (11). PA1 wherein R1 is at least one element selected from rare earth elements (including Y); T1 is at least one element selected from the group consisting of Ca, Ti, Zr and Hf; M6 is at least one element selected from the group consisting of Co, Mn, Fe, Al, Ga, Zn, Sn, Cu, Si, B, Nb, W, Mo, V, Cr, Ta, P and S; and a, b, x and z are respectively a number satisfying conditions: ##EQU3## PA1 wherein R1 is at least one element selected from rare earth elements (including Y); T1 is at least one element selected from the group consisting of Ca, Ti, Zr and Hf; M6 is at least one element selected from the group consisting of Co, Mn, Fe, Al, Ga, Zn, Sn, Cu, Si, B, Nb, W, Mo, V, Cr, Ta, P and S; and a, b, x and z are respectively a number satisfying conditions: ##EQU4## PA1 wherein R1 is at least one element selected from rare earth elements (including Y); T1 is at least one element selected from the group consisting of Ca, Ti, Zr and Hf; M6 is at least one element selected from the group consisting of Co, Mn, Fe, Al, Ga, Zn, Sn, Cu, Si, B, Nb, W, Mo, V, Cr, Ta, P and S; and a, b, x and z are respectively a number satisfying conditions: ##EQU5##
Among them, the rare earth element type hydrogen-absorbing alloy represented by the aforementioned type (1) is now put to practical use as an electrode material. However, the discharge capacity of the alkaline battery comprising this electrode material now reaches to as high as 80% or more of the theoretical capacity, so that any further increase in discharge capacity would be difficult.
By the way, the rare earth element-Ni based intermetallic compound represented by the aforementioned type (1) includes many number of compounds other than an AB.sub.5 type compound (A=a metal element which is capable of exothermically reacting with hydrogen, and B=another kind of metal). For example, Mat. Res. Bull., 11, (1976) 1241 describes that an intermetallic compound containing a larger quantity of rare earth element as compared with the AB.sub.5 type compound is capable of absorbing a larger quantity of hydrogen in the vicinity of normal temperature as compared with the AB.sub.5 type compound. It is also reported that a magnesium-rare earth element based alloy, which is a magnesium-substituted rare earth-Ni based alloy, is capable of absorbing a large quantity of hydrogen gas (Y. Ohsumi, "Soda and Chlorine", 34, 447 (1983)).
It is pointed out by H. Oesterreicher et al in J. Lee-Common Met, 73,339 (1980) that La.sub.1-x Mg.sub.x Ni.sub.2 type alloys for example among the alloys having such compositions are accompanied with a problem that the hydrogen-releasing rate thereof is very low due to the high stability thereof to hydrogen.
There is also a report on a PuNi.sub.3 type hydrogen-absorbing alloy having a composition of Mg.sub.2 LaNi.sub.9, which was made by K. Kadir et al as described in a summary of lecture in the 120th Spring Meeting of Japan Metallic Society, p.289 (1997).
However, the magnesium-rare earth element based alloys having the aforementioned compositions are accompanied with a problem that even though the quantity of hydrogen absorption in a gaseous phase is large, the electrode comprising this alloy scarcely works in an alkaline electrolyte at normal temperature.
Japanese Patent Unexamined Publication S/62-271348 discloses a hydrogen absorption electrode comprising a hydrogen-absorbing alloy represented by a general formula Mm.sub.1-x A.sub.x Ni.sub.a Co.sub.b M.sub.c, while Japanese Patent Unexamined Publication S/62-271349 discloses a hydrogen absorption electrode comprising a hydrogen-absorbing alloy represented by a general formula La.sub.1-x A.sub.x Ni.sub.a Co.sub.b M.sub.c.
However, a metal oxide-hydrogen secondary battery comprising any of these hydrogen absorption electrodes is low in discharge capacity and short in charge/discharge cycle life.
Further, PCT Re-Publication No. WO97/03213 discloses a hydrogen absorption electrode containing a hydrogen-absorbing alloy having a composition represented by a general formula (i); (R.sub.1-x L.sub.x)(Ni.sub.1-y M.sub.y).sub.z, a specific antiphase boundary and a LaNi.sub.5 crystal structure. This hydrogen-absorbing alloy is manufactured by allowing a melt of the alloy represented by the general formula (i) to drop on the surface of a roll, whereby cooling and solidifying the melt under cooling conditions: 50 to 500.degree. C. in supercooling temperature and 1,000 to 10,000.degree. C./sec. in cooling rate, thus obtaining flakes having a thickness of 0.1 to 2.0 mm, which is then heat-treated. This publication also mentions that if the aforementioned manufacturing conditions are not met, the resultant alloy may have two phases, i.e. a LaNi.sub.5 type crystal phase and a Ce.sub.2 Ni.sub.7 type crystal phase, and hence it is impossible to obtain an alloy constituted by the LaNi.sub.5 type crystal phase.
However, a metal oxide-hydrogen secondary battery, which comprises a negative electrode containing this hydrogen-absorbing alloy having a composition represented by the general formula (i), a specific antiphase boundary and a LaNi.sub.5 crystal structure, is accompanied with a problem that not only the discharge capacity but also the cycle life thereof are not satisfiable.